1. Field of the Invention
This invention relates generally to the field of mooring systems for offshore tanker loading/offloading facilities. In particular, the invention relates to soft yoke mooring systems which provide a resilient restoring force for vessels moored to a fixed tower.
2. Description of the Prior Art
Soft yoke mooring systems which use heavy counterweights to provide a restoring force against vessel offset, perform well under moderate environmental conditions. Such systems also perform satisfactorily under fairly severe environmental conditions as long as wind, wave and current directions are nearly collinear.
However, under strong cross current conditions (called "crossed sea" conditions), the moored vessel will be pushed into a quartering or near broadside orientation with respect to the wave direction. The resulting yaw and sway motions of the vessel will, in turn, cause lateral oscillations of the heavy counterweights of the prior soft yoke mooring systems. For pendant lengths between 10 m and 20 m, the natural lateral oscillation period of the yoke and counterweight will be in the order of 6 to 9 seconds. This will often coincide with prevailing wave periods, causing very large counterweight oscillations due to resonance amplification. In many cases, this resonance problem may be unacceptable.
For a submerged yoke system, the fluid drag resistance of the seawater may dampen the lateral oscillations significantly. On the other hand, the submerged counterweights will also be subject to direct excitation by the wave action.
3. Identification of Objects of the Invention
It is a primary object of this invention to provide an improved soft yoke mooring system which overcomes the disadvantage of prior suspended counterweight systems.
Another object of this invention is to provide a soft yoke mooring system which provides superior performance under "crossed sea" conditions as compared to suspended counterweight systems.
Still another object of the invention is to provide a new soft yoke mooring system, which is cost effective, and competitive with prior suspended counterweight systems, especially under severe environmental conditions.